Semiconductor integrated circuit and method for desigining the same

ABSTRACT

A semiconductor integrated circuit according to one embodiment includes a plurality of flip-flop groups configured by dividing a plurality of flip-flops, connected in series, for carrying out a serial operation of serially transferring data, a continuous signal determination circuit configured to output a first signal if outputs of the flip-flops contained in the flip-flop group match, and output a second signal in other cases; and a clock gating circuit configured not to provide a clock signal when receiving the first signal and to provide a clock signal when receiving the second signal with respect to the flip-flops other than a head of the flip-flop group.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2011-068363, filed on Mar. 25,2011; the entire contents of which are incorporated by reference herein.

FIELD

Embodiments described herein relate generally to a semiconductorintegrated circuit and a method for designing the same.

BACKGROUND

In recent years, higher integration and higher speed of the LSI (LargeScale Integrated circuit) are progressing. However, a strong restrictionis imposed on the number of pins to be assigned to the power supply dueto the limitation on the number of pins of the entire LSI. Thus, a greatnumber of circuit blocks share the power supply wiring. As a result,lowering of power supply voltage and rising in ground potential mayoccur during the operation of the LSI. Furthermore, false operation ofthe LSI may occur in the worst case. Such so-called IR drop problem isbecoming more serious with progress in higher integration and higherspeed of the LSI.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing one part of configuration of a semiconductorintegrated circuit according to one embodiment of the present invention;

FIG. 2 is a view showing one example of a state of clock gatingaccording to one embodiment of the present invention;

FIG. 3 shows one example of a histogram for obtaining a maximumeffective data continuing number;

FIG. 4 is one example of a graph showing an effective clock gating ratewith respect to a number of grouped elements; and

FIG. 5 is a flowchart showing a method for designing a semiconductorintegrated circuit according to one embodiment of the present invention.

FIG. 6 is one example of the DFT information of the scan chain.

FIG. 7 is one example of the DFT information after grouping.

DETAILED DESCRIPTION

A semiconductor integrated circuit according to one embodiment of thepresent invention includes a plurality of flip-flop groups configured bydividing a plurality of flip-flops connected in series for performing aserial operation of serially transferring data, and a continuous signaldetermination circuit for outputting a first signal if the outputs ofthe flip-flops in the flip-flop group match and otherwise outputting asecond signal, and further includes a clock gating circuit which doesnot provide a clock signal when receiving the first signal and providesa clock signal when receiving the second signal with respect to theflip-flop other than the head of the flip-flop group.

The embodiment of the present invention will be hereinafter describedwith reference to the drawings. In each figure, the same referencenumerals are denoted for the configuring elements having the samefunctions and the detailed description of the configuring element of thesame reference numeral will not be repeated.

FIG. 1 shows one part of a configuration of a semiconductor integratedcircuit 100 according to a first embodiment of the present invention. Asshown in FIG. 1, the semiconductor integrated circuit 100 includes acombinational logic unit 101 to be scan tested, a flip-flop group 11,and a continuous signal determination circuit 130.

The semiconductor integrated circuit 100 can switch between a normaloperation mode and a scan test mode (shift operation mode) by a testmode signal (TM). In other words, the mode is the normal operation modeof operating normally when TM=0, and is the scan test mode of performingthe scan test when TM=1.

The combinational logic unit 101 is configured by combiningcombinational logic circuits such as an AND circuit, an OR circuit, aninverter, and a buffer. The combinational logic unit 101 is configuredto realize a predetermined logic function at the time of the normaloperation with a flip-flop and a latch shown in FIG. 1.

The flip-flop group 11 includes flip-flops (scan F/F) 111 to 114 andmultiplexers 121 to 124. The flip-flops 111 to 114 are connected inseries through the multiplexers 122 to 124, and output to thecombinational logic unit 101.

As shown in FIG. 1, the multiplexers 121 to 124 have two inputterminals, where one input terminal is connected to the combinationallogic unit 101 and the other input terminal is connected to a scan-interminal (SI) or the flip-flop of the previous stage. A test mode signalis used for a select signal of the multiplexers 121 to 124.

Therefore, in the normal operation mode, the data output from thecombinational logic unit 101 is stored in the flip-flops 111 to 114through the multiplexers 121 to 124. In the scan test mode, the data ofthe scan-in terminal (or output of flip-flop of previous stage (notshown)) and the data output from the flip-flops 111 to 113 are stored inthe flip-flops 111 to 114 through the multiplexers 121 to 124. In otherwords, in the scan test mode, the flip-flops 111, 112, . . . areconnected in series, where the output (Q or SO (Scan Out) which isbuffered output of Q) of the respective flip-flop is input to theflip-flop of the next stage. Therefore, the data stored in theflip-flops 111 to 114 are shifted to the flip-flop of the next stageevery clock cycle. The flip-flop group 11 has been described herein, butother flip-flop groups after the flip-flop group 12 are also similar.

The flip-flop group 12 is connected behind the flip-flop group 11. Theflip-flop group 12 includes flip-flops 211 to 214 and multiplexers 221to 224 (flip-flops 212 to 214 and multiplexers 222 to 224 are notshown). As shown in FIG. 1, the flip-flop group 12 includes theflip-flop 211, which is connected to the flip-flop 114 at the tail ofthe flip-flop group 11 through the multiplexer 221, as the headflip-flop.

Although not shown, flip-flop groups 13, 14, . . . are sequentiallyconnected following the flip-flop group 12.

The flip-flop groups 11, 12, . . . configure a scan chain capable ofcontrolling and observing the flip-flops 111, 112, . . . using thescan-in terminal and the scan-out terminal of the semiconductorintegrated circuit 100 at the time of the scan test mode. The scan chainis configured exterior to the combinational logic unit 101. The scanchain includes a plurality of flip-flops 111 to 114, 211, . . .connected in series, and carries out a serial operation of seriallytransferring data. The flip-flop groups 11, 12, . . . are obtained bydividing the scan chain so that each has a predetermined number offlip-flops (four in the present embodiment).

As shown in FIG. 1, the output terminals (Q) of the flip-flops 111, 112,. . . are connected to the combinational logic unit 101, so that theoutputs of the flip-flops 111, 112 are input to the combinational logicunit 101 at the time of the normal operation mode.

The continuous signal determination circuit 130 outputs “0” if theoutputs (Q output or SO output) of the flip-flops 111 to 114 in theflip-flop group 11 match, that is, if the outputs of the flip-flops 111to 114 are all “0” or all “1”, and outputs “1” in other cases.

As shown in FIG. 1, the continuous signal determination circuit 130includes AND gates 131 to 133, OR gates 134 to 136, an inverter 137, anda negative OR (NOR) gate 138. The continuous signal determinationcircuit 130 is configured to take the negative logical sum of thelogical product (AND) and the inversion (NOT) of the logical sum (OR)for the output signals of the flip-flops 111 to 114 of the flip-flopgroup 11.

More specifically, the output of the AND gate 131 and the output of theinverter 137 are input to the NOR gate 138 to be output to a clockgating control line 103. The output of the AND gate 131 is “1” only whenthe outputs of the flip-flops 111 to 114 are all “1”. On the contrary,the output of the inverter 137 is “1” only when the outputs of theflip-flops 111 to 114 are all “0”. Therefore, the output of the NOR gate138 is “0” only when the outputs of the flip-flops 111 to 114 are all“1” or “0”.

A continuous signal determination circuit 230 having a configurationsimilar to the continuous signal determination circuit 130 is alsoarranged with respect to the flip-flop group 12. A circuit fordetermining whether or not the outputs of the flip-flops of the relevantflip-flop group match is arranged with respect to the flip-flop groupafter the flip-flop group 12.

As shown in FIG. 1, the semiconductor integrated circuit 100 furtherincludes multiplexers 141, 142, and clock gating circuits 151, 152.

The multiplexers 141, 142 respectively outputs an enable signal of theclock gating circuits 151, 152, and uses the test mode signal for theselect signal.

The multiplexer 141 outputs the signal of the clock gating control line102 when TM=0 (normal operation mode), and outputs the signal of theclock gating control line 103 when TM=1 (scan test mode). Themultiplexer 142 outputs the signal of the clock gating control line 102when TM=0, and always outputs the signal “1” when TM=1.

The control signal for clock gating carried out in the normal operationmode is provided to the clock gating control line 102. The signal outputfrom the continuous signal determination circuit 130 is provided to theclock gating control line 103.

The clock gating circuits 151, 152 are circuits configured to performclock gating (disabling of clock signal). For example, the clock gatingcircuits 151, 152 are respectively configured using the AND gate of twoinputs.

As shown in FIG. 1, the clock gating circuit 151 enables/disables theclock signal with respect to the flip-flops 112 to 114, and the clockgating circuit 152 enables/disables the clock signal with respect to theflip-flop 111. The clock gating circuit 152 also enables/disables theclock signal with respect to the flip-flop at the head of the flip-flopgroup after the flip-flop group 12.

The clock gating circuit 151 has a clock signal (CLK) input from oneterminal, and a signal output from the multiplexer 141 is input from theother terminal (enable input terminal). The clock gating circuit 151outputs the CLK signal when the signal output from the multiplexer 141is “1”, and does not output the clock signal when the signal output fromthe multiplexer 141 is “0”.

The clock gating circuit 152 has a clock signal input from one terminal,and a signal output from the multiplexer 142 is input from the otherterminal (enable input terminal). The clock gating circuit 152 outputsthe CLK signal when the signal output from the multiplexer 142 is “1”,and does not output the clock signal when the signal output from themultiplexer 142 is “0”.

The multiplexer 241 has a function similar to the multiplexer 141, andoutputs an enable signal of the clock gating circuit 251. The clockgating circuit 251 functions similar to the clock gating circuit 151,and enables/disables the clock signal with respect to the flip-flopsother than the flip-flop 211 at the head of the flip-flop group 12.

According to the configuration described above, in the normal operationmode (TM=0), the signal of the clock gating control line 102 is input tothe enable input terminals of the clock gating circuits 151, 152 throughthe multiplexers 141, 142. As a result, the flip-flops 111 to 114 areclock gating controlled by the signal of the clock gating control line102. This is similar in the flip-flop groups after the flip-flop group12. Therefore, all the flip-flops configuring the scan chain are clockgating controlled by the signal of the clock gating control line 102 inthe normal operation mode.

In the scan test mode (TM=1), the continuous signal determinationcircuit 130 outputs “0” if the data stored in the flip-flops 111 to 114are all “0” or “1”. The “0” signal is input to the enable input terminalof the clock gating circuit 151 through the multiplexer 141. As aresult, the flip-flops 112 to 114 are gated. The clock gating control ofthe flip-flops during the scan test mode is carried out for everyflip-flop group.

The multiplexer 142 always outputs the signal “1” during the scan testmode, and hence “1” is input to the enable input terminal of the clockgating circuit 152. The clock signal is thus always enabled, i.e.,provided to the flip-flop 111, and the flip-flop 111 is not clock gatingcontrolled during the scan test mode. The reason why the flip-flop 111is always operated during the scan test mode is that the flip-flop 111needs to detect the toggle (signal change of “1”→“0” or “0”→“1”) of theinput signal to the flip-flop group 11.

As described above, at the time of the scan test mode, the flip-flop(111, 211, etc.) at the head of each flip-flop group is always operated,and the other flip-flops are clock gating controlled by the signal ofthe clock gating control line 103.

As a specific operation example, consider a case where “1” iscontinuously shift input to the flip-flop group and “1” is stored in allthe flip-flops 111 to 114 in the scan test mode. In this case, theoutput of the continuous signal determination circuit 130 becomes “0”,and “0” is input to the enable input terminal of the clock gatingcircuit 151, whereby the clock signal (pulse) to the flip-flops 112 to114 is disabled (not provided).

A state in which the clock signal to the flip-flops 112 to 114 isdisabled is continued as long as “1” is shift input to the flip-flop111.

When “0” is transferred to the input terminal (SI) of the head flip-flop111 thereafter, “0” is input to the flip-flop 111 in the next clock. Inthis case, the continuous signal determination circuit 130 outputs “1”and hence “1” is input to the enable input terminal of the clock gatingcircuit 151. As a result, the clock signal (pulse) to the flip-flops 112to 114 is enabled (provided), and the flip-flops 112 to 114 are able toshift the input data.

The operation described above is similarly carried out when “0” iscontinuously shift input to the flip-flop group 11 and “0” is stored inall the flip-flops 111 to 114. The clock signal to the flip-flops 112 to114 is disabled (not provided) as long as “0” is input to the flip-flop111. When “1” is stored in the head flip-flop 111, the clock signal isenabled and the flip-flops 112 to 114 are able to shift the input data.

As can be seen from the description above, when data in which “1” or “0”is continued for N bits (N≧4) is shift input to the flip-flop group 11,“1” or “0” is stored in all four flip-flops 111 to 114 in a certaincycle (time). In the next cycle, the clock signal (pulse) to theflip-flops 112 to 114 other than the head flip-flop 111 is disabled (notprovided). More specifically, the clock signal to the flip-flops 112 to114 is disabled during the (N−3) cycle. As a result, the powerconsumption of the flip-flop group 11 in the scan test mode can bereduced. Furthermore, the data shift output from the flip-flop group 11is serial transferred to the flip-flop group after the flip-flop group12 so that the power consumption can also be reduced for the flip-flopgroups after the flip-flop group 12.

The state of clock gating will be visually described using FIG. 2. FIG.2 shows a state of clock gating occurrence when the data in which “1”and “0”, which continue for seven bits, alternately appear is shiftinput to the scan chain. The scan chain is configured by three flip-flopgroups (Gr1, Gr2, and Gr3). Similar to the flip-flop group 11 describedabove, each flip-flop group Gr1, Gr2, and Gr3 includes four flip-flops(scan F/F) connected in series.

In FIG. 2( a), the horizontal direction to the right indicates the shiftdirection of the data stored in the flip-flop, and the verticaldirection downward indicates time in units of cycles. FIG. 2( a) showsten cycles (T1, T2, . . . , T10). The scan shift operation isrepresented such that each data moves diagonally downward toward rightfor every cycle.

In FIG. 2( a), the portion surrounded with a thick frame shows that theclock gating is effective (i.e., clock signal is not enabled). The clockis always supplied to the flip-flops at the head of the flip-flop groupsGr1, Gr1, Gr3 since they are not target of clock gating.

FIG. 2( b) shows the enable signal of the clock gating control in eachflip-flop group for every cycle. The enable signal is an output signalof the continuous signal determination circuit, and is automaticallygenerated according to the shift input data.

As apparent from FIGS. 2( a) and 2(b), the enable signal is determinedone cycle before the clock gating is carried out. Thus, the clock gatingis made effective in the cycle after the cycle in which the values ofthe flip-flops configuring the flip-flop group match.

In the case shown in FIG. 2, the effective clock gating rate defined asa value obtained by dividing the sum of the cycles in which the clockgating is carried out by all the cycles is 45% (=54/120).

As described above, in the present embodiment, the clock signal (pulse)to the flip-flops other than head of the flip-flop group is disabled(not provided) according to the data shift input to the scan chain.Therefore, according to the present embodiment, the power consumption ofthe scan chain at the time of the scan test can be lowered.

Furthermore, according to the present embodiment, the IR drop can bealleviated since the number of flip-flops operating in the scan testmode is reduced.

The IR drop may suddenly occur during the scan test operation due tomuch more toggles compared to those during the normal operation.According to the present embodiment, however, the problem of so-calledoverkill in which the semiconductor integrated circuit that can beconsidered as a non-defective product is determined as a defectiveproduct in the shipment test can be alleviated since the IR drop can bealleviated. Consequently, according to the present embodiment, the yieldof the semiconductor integrated circuit is enhanced.

In the description made above, the number of flip-flops (hereinafterreferred to as “number of elements” or “number of grouped elements”)contained in the flip-flop group is four, but is not limited thereto.The number of grouped elements becomes the unit of clock gating duringthe shift operation. The number of grouped elements is preferablydetermined so that the effective clock gating rate becomes the largestaccording to the data shift input to the scan chain. This will bedescribed in detail below.

The test pattern input in the scan chain is generated by an automatictest pattern generation tool (hereinafter referred to as “ATPG tool”).The ATPG tool generates a test pattern after grasping the informationrelated to all the scan chains inserted into the logical network of theLSI. The recent ATPG tool has a function (toggle reducing function) ofgenerating a test pattern with smaller number of toggles. The effectiveclock gating rate can be further enhanced by utilizing the suchfunction.

First, a histogram showing the number of occurrences for every number oftimes (cycles) the same data continues (hereinafter referred to as “samedata continuing number”) is created for the test pattern generated bythe ATPG tool. The histogram may be created for the entire test patternor may be created for the data pattern obtained by sampling the testpattern at an appropriate percentage.

FIG. 3 shows one example of a created histogram. In this example, thenumber of occurrences is the most when the same data continuing numberis 1, and the number of occurrences decreases as the same datacontinuing number increases to 2 to 5. With respect to the same datacontinuing number 6, 7, the number of occurrences increases by thetoggle reducing function of the ATPG tool. In same continuing datanumber of 8 or more, the number of occurrences decreases as the samedata continuing number increases.

The same data continuing number in which the product of the same datacontinuing number and its number of occurrences becomes maximum(hereinafter referred to as “maximum effective data continuing number”)is acquired. The maximum effective data continuing number n is aparameter that becomes a basis in optimizing the effective clock gatingrate. FIG. 3 also shows the histogram of the maximum effective datacontinuing number. In this example, the product of the same datacontinuing number and its number of occurrences becomes maximum when thesame data continuing number is 7, and hence the maximum effective datacontinuing number n is 7.

Now, consider representing the effective clock gating rate using themaximum effective data continuing number n and the number of groupedelements m. First, the effective clock gating rate in a case the numberof grouped elements and the maximum effective data continuing number areequal (i.e., n=m) will be described.

In this case, the clock signal to the m−1 (i.e., n−1) flip-flops otherthan the head of the flip-flop group is disabled only once in n cycles.The effective clock gating rate R_CG is thus expressed with equation(1).

$\begin{matrix}{{R\_ CG} = {{\left\{ {N_{SC} \cdot \frac{n - 1}{n} \cdot \frac{N}{n}} \right\} \text{/}\left( {N_{SC} \cdot N} \right)} = \frac{n - 1}{n^{2}}}} & (1)\end{matrix}$

Here, Nsc: length of scan chain (total number of flip-flops configuringscan chain), N: shift operation frequency (number of bits to shift perunit time (one cycle)), n: maximum effective data continuing number.

As apparent from equation (1), the effective clock gating rate R_CGmonotonously decreases as the maximum effective data continuing number nbecomes larger. Therefore, if the maximum effective data continuingnumber n is the number of grouped elements m (i.e., n=m), thecharacteristic in that the toggle is small (i.e., maximum effective datacontinuing number n is large) in the test pattern generated by the ATPGtool cannot be exhibited.

Consider having the number smaller than the maximum effective datacontinuing number n as the number of grouped elements m. In this case(i.e., n>m), the effective clock gating rate R_CG(m) is expressed withequation (2) as a function of m.

$\begin{matrix}{{{R\_ CG}(m)} = {{\left\{ {N_{SC} \cdot \frac{m - 1}{m} \cdot \frac{N\left( {n - m + 1} \right)}{n}} \right\} \text{/}\left( {N_{SC} \cdot N} \right)} = \frac{\left( {m - 1} \right)\left( {n - m + 1} \right)}{nm}}} & (2)\end{matrix}$

Here, Nsc: length of scan chain, N: shift operation frequency, n:maximum effective data continuing number, m: number of grouped elements.

Differentiating equation (2) with m, equation (3) is obtained.

$\begin{matrix}{\frac{{dr\_ CG}(m)}{dm} = {- \frac{m^{2} - \left( {n + 1} \right)}{{nm}^{2}}}} & (3)\end{matrix}$

Solving dR_CG(m)/dm=0, equation (4) is obtained.

m=√{square root over (n+1)}  (4)

Therefore, the number of grouped elements m_best that gives the highesteffective clock gating rate with respect to the maximum effective datacontinuing number n is obtained with equation (5).

m_best=Nearest_int(√{square root over (n+1)})  (5)

Here, the function Nearest_int is a function returning a nearest integerfrom an argument.

According to equation (5), for example, m_best=3 when n=7, and m_best=4when n=12.

FIG. 4 is a graph in which the effective clock gating rate R_CG isplotted with respect to the number of grouped elements m when themaximum effective data continuing number n is between 2 and 20. Thebroken line in FIG. 4 shows the trajectory of the number of elementsm_best that gives the maximum effective clock gating rate with respectto the maximum effective data continuing number n. Thus, the value ofthe number of elements m_best is proportional to the square root of themaximum effective data continuing number n, and hence graduallyincreases with respect to increase in the maximum effective datacontinuing number n.

As the value of m becomes smaller, the additional hardware such as thecontinuous signal determination circuit 130 and the clock gating circuit151 increases. Therefore, the number of grouped elements may bedetermined in view of the trade off between the increase in hardwarescale and the effect (alleviation of IR drop etc.) obtained at lowerpower consumption.

As apparent from FIG. 3, (same data continuing number)×(number ofoccurrences) may not necessarily become considerably large only for aspecific same data continuing number. In other words, (same datacontinuing number)×(number of occurrences) are assumed to often becomevalues approaching each other for a plurality of same continuing datanumbers. Consideration is made on such an aspect and on the fact thatthe additional hardware amount increases as the number of groupedelements becomes smaller. One of the plurality of same continuing datanumbers may be determined as the maximum effective data continuingnumber n, and the number of grouped elements may be calculated withequation (5) with respect to such maximum effective data continuingnumber n. For instance, in the case of FIG. 3, the number of groupedelements may be obtained by substituting n=8 to equation (5). Therefore,the increase in hardware can be suppressed compared to when n=7, and thebalance between the reduction effect of the power consumption and thehardware amount can be enhanced.

The arrows A, B, C in FIG. 4 show the range of the effective clockgating rate with respect to the number of grouped elements m=3, 4, 5,when the maximum effective data continuing number n=5 to 12 is assumed.If the number of grouped elements is 3, the effective clock gating ratehighest on average is obtained. However, the effective clock gating ratein a practically tolerable range can be obtained even if m=4, 5 wherethe additional hardware amount is less than m=3 by taking intoconsideration that the possibility of n>10 is actually significantlysmall.

Furthermore, a sufficiently high effective clock gating rate is obtainedfor the number of grouped elements 2 or 6 depending on the maximumeffective data continuing number. In general, the number of groupedelements m is preferably in a range of m_best−1≦m≦m_best+3 with respectto an integer m_best obtained with equation (5).

In any case, the number of grouped elements m desirably select anappropriate value in view of the trade off between the reduction effectof the power consumption and the additional hardware amount, or thelike.

The number of grouped elements is not limited to one, and a plurality ofvalues may be used for the number of grouped elements so that the scanchain can be divided into a plurality of flip-flop groups. For instance,the number of grouped elements of the flip-flop group 11 may be 4, andthe number of grouped elements of the flip-flop group 12 may be 3.

The additional circuit for realizing lower power consumption such as thecontinuous signal determination circuit 130 does not necessarily need tobe inserted with respect to all the flip-flops configuring the scanchain. In other words, the additional circuit may be inserted only onthe portion where the problem of IR drop and the like are the largest asa result of analyzing the operation of the semiconductor integratedcircuit by the test pattern.

In the internal configuration of the continuous signal determinationcircuit 130, the gate element (AND gate and OR gate) all have two inputsin order to suppress the increase of the signal wiring in FIG. 1, butthis is not the sole case. For instance, if it is effective to suppressthe area of the gate element rather than to increase the signal wiringto suppress the area of the semiconductor integrated circuit, thecontinuous signal determination circuit may be preferably configuredusing a multi-input gate element having three or more inputs. Thelogical product (AND) may be replaced with negative logical sum (NOR)and the logical sum (OR) may be replaced with negative logical product(NAND) by appropriately using inverted signal of the Q output or the SOoutput in place of such Q output or the SO output to configure thecontinuous signal determination circuit.

If the continuing number of “1” is significantly greater than thecontinuing number of “0” as a result of analyzing the test pattern, theOR gates 134 to 136 and the inverter 137 of the continuous signaldetermination circuit 130 may be deleted, and only the continuation of“1” may be detected. In this case, the input terminal of the NOR gate138 connected to the inverter 137 is grounded. In other words, thecontinuous signal determination circuit 130 is configured to output avalue obtained by inverting the value obtained by taking the logicalproduct for the output signals of all the flip-flops of the flip-flopgroup. The hardware amount of the continuous signal determinationcircuit 130 is thereby reduced to about half.

Similarly, if the continuing number of “0” is significantly greater thanthe continuing number of “1” as a result of analyzing the test pattern,the AND gates 131 to 133 of the continuous signal determination circuit130 may be deleted, and only the continuation of “0” may be detected. Inthis case, the input terminal of the NOR gate 138 connected to the ANDgate 131 is grounded. In other words, the continuous signaldetermination circuit is configured to output a value obtained by takingthe logical sum for the output signals of all the flip-flops of theflip-flop group.

Various types of logical gate circuits (AND, OR, etc.) configuring thecontinuous signal determination circuit may be replaced with apre-charging type circuit.

The target of dividing into a plurality of flip-flop groups is notlimited to the scan chain, and a general shift register inside thesemiconductor integrated circuit 100 may serve as a target. The shiftregister may be divided into a plurality of flip-flop groups, and eachflip-flop group may include the continuous signal determination circuit130 and the like, so that the IR drop can be suppressed in the normaloperation.

The method for designing the semiconductor integrated circuit 100according to the present embodiment will now be described along theflowchart of FIG. 5. FIG. 5 shows the procedure for designing asemiconductor integrated circuit inserted with a lower power consumptiontest circuit (hereinafter also referred to as “LP test circuit”).

(1) First, the design for testability (DFT) of inserting a DFT circuitis performed on a logical network of the semiconductor integratedcircuit for realizing the normal operation (step S1). The logicalnetwork herein is configured by basic-cells such as combinational logicgates, flip-flops and latches as the target of the ATPG, and macro-cellssuch as CPU (Central Processing Unit) cores, memories, A-D(Analog-Digital) converters, D-A converters, and PLL (Phase-Locked Loop)circuits. Some macro-cells may include scan chains, also being thetarget of the ATPG.

The information used in step S1 includes test restriction which is thespecification of the scan test, the basic-cells including theflip-flops, latches and the combinational logical gates configuring thecombinational logic, and the logical network showing the logical circuitconfiguration of the basic-cells and macro-cells. The logical network iscreated by logical synthesis from the high level description by the RTL(Register Transfer Level), or the like.

The logical network after the DFT insertion (i.e., logical networkinformation inserted with the DFT circuit) and the DFT information arecreated by executing step S1. The DFT information shows the connectionstate of the flip-flop configuring the scan chain.

FIG. 6 shows one example of the DFT information of the scan chain. InFIG. 6, the “F/F No.” column at the left end shows the connecting orderof the flip-flop (scan F/F). The “Clock” column shows the name of theclock that provides the clock signal to the flip-flop. The “Cell_name”column shows the name of the cell of the flip-flop. The “SI” columnshows the name of the input terminal (scan-in terminal) of the flip-flopcontaining the hierarchical information of the logical network of theLSI. The “SO” column shows the name of the output terminal (scan-outterminal) of the flip-flop containing the hierarchical information ofthe logical network of the LSI.

The DFT information shows that the SO terminal of the flip-flop in acertain row is connected to the SI terminal of the flip-flop in the nextrow in the test mode. For instance, the output terminal(top.abcd.U3029.Q) of the F/F No. 1 is connected to the input terminal(top.abcd.U3941.SI) of the F/F No. 2.

(2) The test pattern is then generated using the ATPG tool (step S2).The information related to the logical network after the DFT insertionand the DFT information, which are both obtained in step S1, are used togenerate the test pattern. The DFT information is used because the ATPGgenerates the test pattern corresponding to the configuration of thescan chain.

The test pattern with as small number of toggles as possible ispreferably generated to enhance the effective clock gating rate and tosuppress the IR drop.

(3) The data array of the test pattern obtained in step S2 is thenanalyzed to acquire the maximum effective data continuing number n. Thenumber of grouped elements is calculated based on the maximum effectivedata continuing number n (step S3). For instance, the number of groupedelements m_best that realizes the highest effective clock gating rate iscalculated by substituting the maximum effective data continuing numbern to equation (5).

The analysis of the test pattern is carried out, for example, bycreating a histogram of the same data continuing number, as describedusing FIG. 3.

The effective clock gating rate may be calculated by substituting thecalculated number of grouped elements m_best to equation (2), and sucheffective clock gating rate may be used as the report information to thedesigner.

(4) The grouping information is added to the DFT information using thenumber of grouped elements m_best calculated in step S3 to create theDFT information after grouping (step S4). Such grouping information isthe information for dividing the flip-flops configuring the scan chainto a plurality of groups.

FIG. 7 shows one example of the DFT information after grouping. As shownin FIG. 7, in the DFT information after grouping, the “Group” columnserving as the grouping information is added to the DFT information.Such “Group” column is described in the form of (group ID number).(intra-group number), and indicates what order in the flip-flop groupthe flip-flop is present. For instance, the information “1.3” of the F/FNo. 3 indicates the third flip-flop of the first flip-flop group.

(5) The scan chain is then divided into a plurality of flip-flop groupsaccording to the DFT information after grouping obtained in step S4, andthe LP test circuit is inserted into each flip-flop group (step S5). Thelogical network (i.e., final logical network) of the semiconductorintegrated circuit inserted with the LP test circuit is thereby created.In other words, in step S5, the logical network is updated so as toinsert the LP circuit to each flip-flop group.

Specifically, the LP test circuit is the continuous signal determinationcircuit 130, 230, . . . in the case of the embodiment shown in FIG. 1.In the case of the semiconductor integrated circuit that carries outclock gating in the normal operation, the multiplexers 141, 142, 241 andthe clock gating circuits 151, 152, 251 are already arranged, and asignal line is merely re-connected for such circuits. In the case of thesemiconductor integrated circuit that does not carry out clock gating inthe normal operation, such circuits are also inserted as the LP testcircuit by the process of the present step. The LP test circuit is acircuit that, when the same data (“0” or “1”) is stored in all theflip-flops configuring the flip-flop group in a certain cycle, disablesthe clock signal (pulse) to the flip-flop other than the head flip-flopof the flip-flops configuring the flip-flop group in the next cycle.

The semiconductor integrated circuit inserted with the LP test circuitis designed through the procedures described above. The description madeabove is for a case where there is one scan chain, but if a plurality ofscan chains are created by the process of step S1, the process of stepS4 is executed for every scan chain. This process may be executed inparallel.

In the semiconductor integrated circuit designed with the methodaccording to the present embodiment, the LP test circuit performs themost effective clock gating on the corresponding flip-flop group in thescan test. The power consumption of the semiconductor integrated circuitin the scan test is thereby reduced, and the IR drop can be alleviated.

In the above description of the method for designing the semiconductorintegrated circuit, the scan chain for the scan test is the target oflower power consumption. A general shift register in the logical networkfor the normal operation may be the target of lower power consumption.The method for designing the semiconductor integrated circuit in thiscase will be described below.

First, the process of extracting a shift register from the logicalnetwork of the semiconductor integrated circuit is carried out, and theshift register information indicating the connection state of theflip-flop configuring the extracted shift register is created. Suchshift register information corresponds to the DFT information describedabove.

The logical simulation of the logical network of the semiconductorintegrated circuit is then carried out to acquire the data to be shiftedon the extracted shift register. Similar to step S3 described above,such data is analyzed to acquire the maximum effective data continuingnumber n. The number of grouped elements is then calculated based on themaximum effective data continuing number. For instance, the number ofgrouped elements that realize the highest effective clock gating rate iscalculated by substituting the maximum effective data continuing numbern to equation (5).

The grouping information for dividing the flip-flops configuring theshift register into a plurality of groups is then added to the shiftregister information using the calculated number of grouped elements,thereby creating the shift register information after grouping. Theshift register information after grouping corresponds to the DFTinformation after grouping.

Lastly, the shift register is divided into a plurality of flip-flopgroups according to the shift register information after grouping, andthe lower power consumption circuit is inserted into each flip-flopgroup to create the final logical network. In other words, the logicalnetwork is updated to insert the lower power consumption circuitto eachflip-flop group.

Such lower power consumption circuit is similar to the LP circuitdescribed above.

The number of grouped elements is not limited to m_best in the twomethods for designing the semiconductor integrated circuit, and the DFTinformation after grouping (or shift register information aftergrouping) may be created using the value of greater than or equal to(m_best−1) and smaller than or equal to (m_best+3).

Furthermore, the DFT information after grouping (or shift registerinformation after grouping) may be created using a plurality of numberof grouped elements.

The LP test circuit or the lower power consumption circuit includes acircuit for taking the logical sum and/or logical product of the outputsof all the flip-flops configuring the flip-flop group.

If the majority of the data passing the shift register is “0” or “1” dueto the specification of the LSI, the lower power consumption circuit tobe inserted may simply take the logical product or the logical sum ofthe outputs of the flip-flops configuring the flip-flop group.Specifically, in the case of the embodiment shown in FIG. 1, the logicalsum signal by the OR gates 134, 135, and 136 may be used as it is as theinput signal to the multiplexer 141 if the majority of the data passingthe shift register is “0”, and the signal inverted from the logicalproduct signal by the AND gates 131, 132 and 133 may be used as theinput signal to the multiplexer 141 if the majority of the data passingthe shift register is “1”. The hardware amount of the lower powerconsumption circuit is thereby reduced to about half.

According to the designing method described above, the power consumptionin the normal operation of the shift register in the logical circuit fornormal operation (e.g., semiconductor integrated circuit 100 of FIG. 1)can be reduced. As a result, the IR drop can be suppressed in the normaloperation.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

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 15. A method fordesigning a semiconductor integrated circuit comprising: creating alogical network after DFT insertion and DFT information indicating aconnection state of flip-flops configuring a scan chain by performing adesign for testability of inserting a DFT circuit on a logical networkof the semiconductor integrated circuit; generating a test pattern usinginformation related to the logical network after the DFT insertion andthe DFT information; acquiring a maximum effective data continuingnumber in which a product of a same data continuing number and number ofoccurrences of the same data continuing number becomes maximum byanalyzing the test pattern; calculating a number of grouped elementsbased on the maximum effective data continuing number; creating DFTinformation after grouping by adding grouping information for dividingthe flip-flops into a plurality of groups to the DFT information usingthe number of grouped elements; and dividing the scan chain into aplurality of flip-flop groups according to the DFT information aftergrouping, and updating the logical network so as to insert a lower powerconsumption test circuit to each flip-flop group, the lower powerconsumption test circuit being configured not to provide a clock signalto the flip-flop other than a head flip-flop of the flip-flopsconfiguring the flip-flop group in the next cycle if same data is storedin all the flip-flops configuring the flip-flop group in a certaincycle.
 16. The method for designing the semiconductor integrated circuitof claim 15, wherein a number in a range of m_best−1≦m≦m_best+3 withrespect to an integer m_best given by equation (2) is used for thenumber of grouped elements,m_best=Nearest_int(√{square root over (n+1)})  (2) where functionNearest_int is a function for returning a nearest integer from anargument, and n is the maximum effective data continuing number.
 17. Themethod for designing the semiconductor integrated circuit of claim 15,wherein the lower power consumption test circuit includes a circuitconfigured to take a logical sum and/or logical product of outputs ofall the flip-flops configuring the flip-flop group.
 18. A method fordesigning a semiconductor integrated circuit comprising: extracting ashift register from a logical network of a semiconductor integratecircuit, and creating shift registerinformation indicating a connectionstate of flip-flops configuring the shift register; acquiring data to beshifted on the shift register by carrying out a logical simulation ofthe logical network; acquiring a maximum effective data continuingnumber in which a product of a same data continuing number and number ofoccurrences of the same data continuing number becomes maximum byanalyzing the data; calculating a number of grouped elements based onthe maximum effective data continuing number; creating shift registerinformation after grouping by adding grouping information for dividingthe flip-flops into a plurality of groups to the shift registerinformation using the number of grouped elements; and dividing the shiftregister into a plurality of flip-flop groups according to the shiftregister information after grouping, and updating the logical network soas to insert a lower power consumption test circuit to each flip-flopgroup, the lower power consumption test circuit being configured not toprovide a clock signal to the flip-flop other than a head flip-flop ofthe flip-flops configuring the flip-flop group in the next cycle if samedata is stored in all the flip-flops configuring the flip-flop group ina certain cycle.
 19. The method for designing the semiconductorintegrated circuit of claim 18, wherein a number in a range ofm_best−1≦m≦m_best+3 with respect to an integer m_best given by equation(3) is used for the number of grouped elements,m_best=Nearest_int(√{square root over (n+1)})  (3) where functionNearest_int is a function for returning a nearest integer from anargument, and n is the maximum effective data continuing number.
 20. Themethod for designing the semiconductor integrated circuit of claim 18,wherein the lower power consumption test circuit includes a circuitconfigured to take a logical sum and/or logical product of outputs ofall the flip-flops configuring the flip-flop group.